Dies for cutting webs to provide lines of weakness for separating portions of a web from other portions of the web are known in the art. See, for example, U.S. Pat. No. 5,117,721. Such dies include a toothed cutting rule mounted on a flat or rotary die board. Instead of a continuous cutting edge, the rule has a series of teeth separated by notches or nicks. When pressed into the material to be cut, the teeth make a series of cuts through the material and leave a corresponding series of bridges, lands or ties there-between, forming the lines of weakness. Portions of the material formed in this manner, e.g., sheet stock, are separated from the adjacent material by rupturing the ties along one or more of these lines of weakness. However, to be commercially acceptable, the edges of the removed material often should have an appearance which, to the naked eye, is substantially the same as one which has been cut with a die having a continuous cutting edge. See, for example, U.S. Pat. No. 5,240,755 and Canadian Patent No. 1,194,517. The cutting rule described in said U.S. Pat. No. 5,117,721 does not meet this requirement because of the shape, size and spacing of the teeth.
One problem in using conventional cutting rule is that when the teeth are forced into the stock, the rule not only cuts through the stock at the cutting edges of the teeth, but also pushes the tie areas therebetween outward, and when the desired, or to be used, part or product is removed from the stock, there are irregular raised areas or bumps along the edges of the desired part. These bumps, which are visible to the naked eye, must be sanded off to give a product formed from the web a more aesthetic appearance. A further problem is that conventional perforating rules have a cross-sectional shape, as illustrated in FIG. 4, which, when cutting, locally stresses the material along the area proximate to the cutting edge of the rule. The stresses, if not adequately distributed, could rupture the ties of the material within the notches between the teeth, particularly where the ties are closely-spaced to one another. When the ties are sufficiently separated, stresses generated by the rule teeth pushing into the web are distributed along the three dimensions of web material, thereby reducing tie rupture. However, by increasing the spacing between ties, i.e. the width of the cuts, the strength of the connection between the final or desired part and adjacent material is reduced, which can be unsatisfactory for subsequent processing of the stock, unless the width of the tie is increased which can make the ties more readily observable when the final or desired part is separated from adjacent material. Accordingly, there is a need for a rule having an improved tooth shape and spacing which reduce the amount of stress on the web material, eliminates or reduces the rupturing of ties even if closely-spaced, and also prevents the formation of visible imperfections at the edges of the part removed from the stock.
Another problem associated with conventional rule is limited bendability and flexibility, particularly if the rule is heat-treated to increase its hardness and durability. The prior art rule shown in cross-section in FIG. 4, for example, has a thickness of approximately 2 points, or 1/36 inch, a metal thickness which, if hardened by heat-treatment, would cause the rule to fracture if bent. Thus, the conventional rule of FIG. 4 may be suitable only for making rectilinear perforations, and rules for making curved or non-linear cuts would either be made of a softer, bendable and less durable material or would require special manufacture. Accordingly, there is a need for rules that have both the durability of hardened rules and the flexibility of softer ones.